The present invention relates to microwave tank-contents level measuring systems, and more particularly to the assembly to be mounted at an upper point of a tank the level of whose contents is to be measured.
In my copending U.S. patent application Ser. No. 692,576, filed Jan. 18, 1985, and entitled "Microwave Tank-contents Level Measuring Assembly With Lens-obturated Wall-opening", now U.S. Pat. No. 4,566,321, there is disclosed and claimed a system which is contained within a housing having an open end obturated by a hyperbolically curved convex-plano fluid impermeable solid dielectric lens providing a seal for the tank opening over which the housing is installed while collimating an outgoing ranging signal and focusing the return signal. A separate removeable housing cover provides access to the electronic components for service while the tank-opening sealing function of the lens is not disturbed. A transmitting-receiving horn is dimensioned and positioned on the axis of the lens with its effective phase center at the operating frequency coinciding with a focal point of the hyperbolic surface.
From a system standpoint, the distance measurement is determined by projecting toward the surface of the tank contents a microwave ranging signal whose frequency is swept back and forth relative to some predetermined center frequency. The radiated signal is reflected, in part, from the surface of the tank-contents and arrives back at the transmitting point after a delay which is a function of the transit time. The returned signal is mixed with a local signal corresponding to the instantaneous signal then being transmitted to produce a difference frequency signal which is representative of the distance to the surface of the tank contents. The difference frequency signal is sinusoidal in nature.
While the assembly described in my aforesaid application solves one major problem encountered with microwave level measuring equipment, namely, the problem of maintaining adequate containment of volatile gases and the like, such equipment suffers from its own peculiar problem. When apparatus as disclosed in my aforesaid application was used to measure distance to a target surface, a strong sinusoidal signal was detected that appeared as if coming from a target about two feet away. Testing the apparatus further, and simulating an empty tank, a weak return signal was produced at a much higher frequency than the strong signal but difficult to detect because of the strong lower frequency signal. On the other hand, when attempting to measure level in a full or near full tank, the desired signal was close in frequency to the strong signal and now difficult to separate from the strong signal.
After careful study and investigation it was discovered that the swept frequency ranging signal was experiencing reflection at the interface between the plano surface of the lens and the tank atmosphere, which reflection, although representing only a small fraction of the radiated energy, was large compared to legitimate returning ranging signals. Consequently, the lens interface reflection made detection of the tank-contents surface extremely difficult.
Dielectric lenses for use as focusing elements for microwave-antenna systems have been known for many years, and the antenna art has been aware of the energy losses that occur as a consequence of lens interface reflections. In an article entitled "Surface Matching of Dielectric Lenses" by E. M. T. Jones and S. B. Cohn, published in the Journal of Applied Physics, Vol. 26, No. 4, April 1955, pp. 452-457, two methods are discussed for cancelling the surface reflections of dielectric lenses. One method employs a reactive wall embedded within the dielectric. The reactive wall can be either of a capacitive nature formed from thin metallic discs distributed in an hexagonal array; or of an inductive nature formed from a wire grid. The correction described in this article is frequency dependent and all of the calculations therein are based upon use of a fixed frequency signal. Neither the Jones et al. article, nor subsequent articles written on the same subject, contain any suggestion that the reactive wall concept can be used advantageously when the signal does not have a constant fixed frequency.